For example, Japanese Patent Application Laid-Open Publication No. 2002-374630 (Patent Document 1), Japanese Patent Application Laid-Open Publication No. 2009-131020 (Patent Document 2), Japanese Patent Application Laid-Open Publication No. 2001-14042 (Patent Document 3) and Japanese Patent Application Laid-Open Publication No. H11-127543 (Patent Document 4) show systems in which an overcurrent at a secondary battery is detected via on-resistance of a FET inserted in a current path thereof to protect against the overcurrent. Specifically, Patent Document 1 shows techniques for maintaining gate voltages of a charge-control switch element and a discharge-control switch element to be constant regardless of a battery voltage, thereby keeping the on-resistance of each switch element constant. The gate voltage is set to be, for example, the possible lowest voltage of the battery voltage. Patent Document 2 shows techniques in which first and second reference FETs having the same characteristics as charge and discharge FETs are provided, the same gate voltage is applied to the first and second reference FETs, and a voltage detected via the on-resistance of the charge and discharge FETs is determined with using a voltage generated via the first and second reference FETs as a reference voltage.
Furthermore, Patent Document 3 shows techniques in which a detection voltage detected via the on-resistance of a FET is compared with a reference voltage generated in a reference-voltage control circuit (voltage reduced in a curve along with increase in a battery voltage), and when the detection voltage exceeds the reference voltage, the FET is driven to be off. Patent Document 4 shows techniques in which a gate voltage of a discharge-control FET is added as an offset voltage to a detection voltage detected via the on-resistance of the FET, and a discharge-current detection voltage obtained by this addition is used to detect an overcurrent. In this manner, when a discharge-control FET having low on-resistance is used, the overcurrent can be detected without applying a current more than necessary to the FET.